This invention relates generally to television receivers and particularly to automatic fine tuning system for use in television receivers.
Recently, numerous systems have provided television receivers with the capability of being electronically tuned to receive any of the 83 allocated television channels. Such systems generally tune to the UHF channels as accurately and easily as to the VHF channels and is therefore highly desirable because of the FCC "equal tuning" rule.
One such system is described in U.S. Pat. No. 3,961,266 entitled "Channel Seeking Tuning System" and assigned to Zenith Radio Corporation. Briefly, that patent discloses a channel tuning system in which the number of a desired channel may be manually entered by a viewer, whereupon the receiver is automatically tuned to a restricted frequency "window" about the nominal carrier frequency of the desired channel. Upon receipt of a carrier signal within that window, a conventional AFC (automatic frequency control) system locks the tuner to the frequency of the incoming carrier.
A related improvement in high quality television receivers is in the use of so-called "true" synchronous detectors to demodulate the video components of a received signal. Such "true" synchronous detectors effectively multiply the IF (intermediate frequency) television signal by a reference oscillator signal to detect the video components. The output of the synchronous detector is a video signal having far less distortion than conventional video detectors.
In view of the present trend toward high quality television receivers and the availability of complex, yet inexpensive electronic integrated circuitry, it is desirable to provide a television receiver which enjoys the advantages of "window" tuning along with synchronous detection of the video signal.